This is the comment that made me think you were suggesting splitting the house code for different device types.
"Another suggestion is NOT to mix different classes of modules (Appliance modules with Lamp modules, and even SoftStart with non-SoftStart modules) on the same HouseCode / UnitCode combination. since each type requires a different set of commands to operate, you may not get the desired results."
Sounds like some folk might set up multiple modules with the same house.unit code, presumably to "group" the devices and save some time (i.e. make changes for the group vs. for each module). Also useful if they have more than 15 modules.
Actually it is for those people that have two or three plug-in lamps in the same room, and they want all of them to come on and go off together. Setting them for the same HouseCode/UnitCode is the best way, since only one command is needed, and all three respond to it. However, if the modules are different types, that won't work too well.
...Although I can't remember why, I put the X-10 Pro PAT01 (RR501) on a different house code ("A" instead of "C" which I use for all other modules). Unfortunately I chose the same house code as the sample room in AHP. I'll change the sample room modules to a different house code just to remove that variable.
When I get around to testing the PAT01, I'll review the instructions. I vaguely remember the unit's manual address setting was just for the lamp plug.
As Brian noted, the outlet on the PAT01 is equivalent to an Appliance module, not a Lamp module (it has no dimming).
If you plan to use any RF remotes, or motion sensors to directly control your lights or appliances, then the PAT01 needs to be on the same HouseCode as the modules you plan to control.
As Brian also noted, the CM15A and PAT01 (or RR501) are both capable of receiving RF signals, and sending them out on the powerline. Depending on your setup, you might not need the PAT501 at all.
The CM15A is notorious for having very poor RF reception range, but there are a number of fixes for that, at least one of which doesn't involve opening up the unit or voiding the warranty. You can do a search for "Antenna Mod" in the forums and find the various options. I'm using the "passive reflector" type of mod on mine, and it works really well for me.